In this article, we theoretically and experimentally study the conversion from a circularly polarized (CP) plane electromagnetic wave into a linearly polarized (LP) transmitted one using anisotropic self-complementary metasurfaces. For this purpose, a metasurface design operable at sub-terahertz frequencies is proposed and investigated in the range of 230-540 GHz. The metasurface is composed of alternating complementary rectangular patches and apertures patterned in an aluminum layer deposited on a thin polypropylene film. Our study shows that the translational symmetry of the metasurface results in unusual and useful electromagnetic properties under illumination with CP radiation beams. In particular, alongside with broadband circular-to-linear conversion, the transmitted wave exhibits a frequency-independent magnitude, while its polarization angle gradually changes with frequency that paves the way for new beam-splitting applications.